/Engine/Source/Runtime/Engine/Public/SceneViewExtension.h
This will be called at the beginning of post processing to make sure that each view extension gets a chance to subscribe to an after pass event.
virtual void SubscribeToPostProcessingPass ( [EPostProcessingPass](API\Runtime\Engine\ISceneViewExtension\EPostProcessingPass) Pass, FAfterPassCallbackDelegateArray & InOutPassCallbacks, bool bIsPassEnabled )